Seesall writes:
>In fact if the contestant chooses door A and Monty never opens C when the car is behind A the chances on B are 100%

That is correct but one third of the time, i.e. when the car is behind door A, Monty also opens door C (i.e., one of the wrong doors). Thus Keith Devlin's column is correct.
Have a Good Day
KJ Ramsey